1. Field of the Invention
The present invention relates to an optical device, a polarizing plate, a diffuser, a diffusing film, a polarizing film, and a liquid crystal image display apparatus, which are capable of suppressing luminance of a liquid crystal display apparatus (liquid crystal image display apparatus) from being lowered by dispersing (diffusing) light, thereby decreasing color variation in different view angles.
The present invention also relates to a liquid crystal image display apparatus, such as a liquid crystal TV, having a vertical-aligned liquid crystal cell in which liquid crystal molecules are vertically aligned under no application of a voltage, for example.
This application is based on Japanese Patent Application Nos. 2006-232137 filed on Aug. 29, 2006, 2006-239955 filed on Sep. 5, 2006, and 2006-262012 filed on Sep. 27, 2006, the contents of which are incorporated herein by reference.
2. Description of Related Art
In the related art, a liquid crystal display apparatus (liquid crystal image display apparatus) provided on a liquid crystal TV, a car navigation system, a PC monitor and so on, includes a liquid crystal panel and a backlight unit which faces the liquid crystal panel and provides light to the liquid crystal panel. The liquid crystal panel includes, for example, a liquid crystal cell including liquid crystal composed of bar-shaped liquid crystal molecules and a pair of substrates having electrodes to apply a voltage to the liquid crystal with the liquid crystal interposed between both substrates, a pair of polarizing plates (analyzer and polarizer) which transmit only unidirectional oscillating light respectively with the liquid crystal cell interposed therebetween, and RGB color filters which are interposed between the polarizing plate (analyzer) which are arranged at a front side (foreside) in which an image of the liquid crystal image display apparatus is displayed and the substrate.
This kind of liquid crystal display apparatus is classified as a TN (Twisted Nematic) type, VA (Vertical Alignment) type, an IPS (In Plane Switching) type, an OCB (Optically Compensated Bend (Optically Compensated Birefringence)) type, etc. based on the difference between alignment directions (orientations) of liquid crystal molecules depending on conditions of application of a voltage. In a TN type liquid crystal display apparatus, liquid crystal molecules are twistedly aligned (twisted orientation) by 90 degrees with an axis perpendicular to the substrates under no application of voltage. In this state, as light (incident light) emitted from a backlight transmits a polarizing plate (polarizer) arranged at a rear side (back side) and travels along the twisted orientation of the liquid crystal molecules, oscillation direction of the light is changed to the same direction as a transmission axis of a polarizing plate (polarizer) at a front side, and transmitted through the polarizing plate at the front side. On the other hand, if a voltage is applied to the liquid crystal display apparatus, the alignment of the liquid crystal molecules is changed in a vertical direction along an electric field. The incident light travels along the liquid crystal molecules aligned in the vertical direction and is intercepted by the polarizing plate at the font side. In this manner, in the TN type liquid crystal display apparatus, display/non-display of an image is controlled based on liquid crystal molecule alignment and light transmission state depending on voltage application conditions.
However, in the TN type liquid crystal display apparatus, since the liquid crystal molecule alignment is in accord with the vertical direction when the voltage is applied, light in the vertical direction with respect to a display plane is completely intercepted, but light in an inclined direction is leaked without being intercepted. Therefore, when the liquid crystal display apparatus is viewed in a straight direction and an inclined direction with respect to the foreside, light transmissions in both directions are different from each other, and accordingly, an image is differently viewed in both directions (that is, a view angle is small). In addition, since the liquid crystal molecules are in the twisted orientation, although polarization is corrected using a retardation film (phase difference film (phase difference optical device): a film having biaxial refractive anisotropy), gradation inversion in one direction occurs.
In a VA type liquid crystal display apparatus, liquid crystal molecules are aligned perpendicular to substrates (vertical orientation) under no application of a voltage, and the alignment direction of the liquid crystal molecules is changed in a horizontal direction parallel to the substrates to transmit light under application of a voltage. Accordingly, the VA type liquid crystal display apparatus has high contrast and can easily reproduce black gradations, as compared to the TN type liquid crystal display apparatus. However, the VA type liquid crystal display apparatus has a problem of color variation in which gamma (gradation characteristic) is varied depending on a view angle (for example, an image looks white when the image is observed in an inclined direction). In addition, in a case in which a retardation film is applied to the VA type liquid crystal display apparatus, an effect of the retardation film can be obtained only when liquid crystal molecules are vertically aligned, but polarization cannot be corrected for halftone. Therefore, the effect can not be obtained for gradation variation depending on the view angle, and particularly, color variation for blue color having a short wavelength (400 nm to 500 nm) in halftone can not be overcome.
On the other hand, for example, a liquid crystal display apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. H10-10513 includes a backlight (surface light source), a liquid crystal panel, a polarizing plate, a phase difference film (retardation film) and a light diffusing layer. This liquid crystal display apparatus has high contrast by suppressing a rise in black level in an inclined direction using the phase difference film and canceling gradation inversion by using the light diffusing layer, and particularly, it is possible to provide wide-viewing display with no gradation inversion in a downward direction.
For example, a liquid crystal display apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-4763 includes a backlight system which parallelizes diffused light using a polarizing device (A) having at least two layers of reflecting polarizers (a) overlapping with each other in a wavelength band of selective reflection of polarization, with a phase difference layer (b) interposed between the polarizers (a), a liquid crystal cell which transmits the parallelized light, polarizing plates (analyzer and polarizer) arranged at both sides of the liquid crystal cell, and a view angle extending layer which is disposed at a viewing side (front side) of the liquid crystal cell and diffuses the transmitted light. In this liquid crystal display apparatus, the backlight system converges emitted light rays into only a view angle region having highest contrast and good color reproducibility. In other words, the liquid crystal display apparatus having high resistance to gradation inversion or color tone variation and a good view angle characteristic is obtained by converging light rays, which are leaked in an inclined direction, into a front direction to average the light rays.
However, in the liquid crystal display apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. H10-10513, color variation (color shift) when viewed in different directions can not be sufficiently suppressed although the view angle can be extended and the gradation inversion can be prevented. In addition, the liquid crystal display apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-4763 is expensive since the backlight system to converge the light rays in the front direction has a complicated structure; and has much loss of light. Moreover, in Japanese Unexamined Patent Application, First Publication No. 2004-4763 disclosing this liquid crystal display apparatus, there is no mention about specified structure of a diffusing film and an effect of color correction, and therefore, it can be considered that the color variation when viewed in different directions can not be sufficiently suppressed.
On the other hand, a diffusing film (antiglare film) having an antiglare function has been put into practical use. However, since this kind of diffusing film has no color correction function, it also can not sufficiently suppress color variation, and assuming that a liquid crystal display apparatus is used for TV, there is a need to provide a separate hard coat layer to protect a front side at which an image is displayed or a separate antiglare layer to prevent reflection of external light, for example. On the other hand, there is a recent keen need to reduce production costs of liquid crystal display apparatuses. Accordingly, such separate provision of a hard coat layer, an antiglare layer, an optical device having only a color correction effect, etc. leads to an increase in the number of manufacturing processes of panel constituent members, which results in high costs due to bad manufacturing yield, and there is a keen need for an optical device having a color correction effect while having both of hard coat and antiglare functions.
In addition, even when a diffusing film is provided at a front side of a liquid crystal panel in order to suppress color shift, the color variation can not be sufficiently suppressed, since the diffusing film can not give a color correction effect. Moreover, since black level luminance at the front side (that is, transmittance at the front side when black is displayed) rises in the entire wide band of wavelength due to light diffusion by the diffusing film, as shown in FIG. 22, a problem occurs in that the black level rises when viewed in an inclined direction.